1. Field of Invention
The invention pertains to the field of electrochemical corrosion chemistry. The corrosion sensor has potential applicability in a wide variety of painted metal structures, including highways and bridges, storage tanks, pipelines, locks and dams, towers, aircraft, vehicles, and ships. The most important benefit of this technology will be increased safety and reliability. The field-operational sensors will allow maintenance inspectors to detect the early stages of coating degradation/corrosion well before serious deterioration has occurred. Maintenance can then be scheduled based on the actual condition of the structure and need not be performed on an elapsed time schedule. Thus, the sensor could also potentially save maintenance cost in three ways:
Allowing an "as required" maintenance schedule PA1 Initiating repairs before it becomes costly to perform them PA1 Providing quantitative data regarding rates, and mechanisms of degradation.
Additionally, the ability to quantify corrosion rates and correlate laboratory and field exposure should become very important in the development and selection of new materials and processes. This added intelligence should enable structure lifetimes to be increased more economically than currently feasible.
In view of the tremendous economic and environmental cost of maintenance of infrastructures, the potential commercial applications of this technology to the construction and the military industries are enormous. Structure or hardware will have an increased lifetime utility with lowered lifetime maintenance cost. The commercial ramifications of electrochemical corrosion sensor development are also important to the insurance industry where the potential economic benefits could be substantial. Additionally, increased safety will be one of the most important benefits of this technology.
2. Description of the Related Art
An atmospheric in-situ sensor has been previously conceived for early detection of corrosion on painted steel..sup.1 FNT .sup.1 Simpson, T. C. Moran, P. J., Hampel, H., Davis, G. D., Shaw, B. A., Arah, C. O., Fritz, T. L., and Zankel, K. L., "Electrochemical Impedance Measurements for Evaluating and Predicting the Performance of Organic Coatings for Atmospheric Exposure", Corrosion Testing and Evaluation: Silver Anniversary Volume, ASTM STP 1000, R. Baboian and S. W. Dean, Eds., American Society for Testing and Materials, Philadelphia, 1990, pp. 397-412.
The prototype sensor (1), (FIG. 1) as developed earlier, was a painted steel coupon approximately 1" by 1". A gold grid electrode (2) was deposited onto the steel coupon (3) by electron beam. This eliminates the need for a remote or counter electrode. Standard Electrochemical Impedance Spectroscopy (EIS) testing requires counter electrode because of the high impedance of the electrolyte. The monitor uses the fact that the gold grid electrode impedance is much lower than that of the coating on a metal. This results in low interfacial impedance between the electrodes and allows for a two electrode testing. This two electrode method enables an in-situ real time analysis of a metal/coating system, previously unavailable. Furthermore, it has been determined that the contact of the gold grid electrode with the coating does not accelerate or initiate corrosion of the coating at the contact point. Electrical lead wire (4) from the painted gold grid, acts as the reference and the counter electrode. Electrical lead wire (5) from the back of the sample acts as the working electrode. Electrical lead wire (6), from the front of the sample, acts as the counter electrode.
This technique was abandoned for a more practical approach using a low adhesive strength tape to lay the pattern of the grid. This technique proved to be simpler in design and allowed for easier application of the paint.
Silver paint was added to the scope of testing to provide another low cost alternative to the gold paint since there was concern that the carbon paint might not give accurate results due to its low electrical conductivity and suspected low abrasion resistance. The values for the measured electrical conductivity of the deposited electrodes are shown in Table 1.
TABLE 1 ______________________________________ Current Resistance of Electrodes Taken from Opposite Corners of the Paint Grid. Cure Temperature Resistance Electrode (C.) (Ohms) ______________________________________ Gold (Au) 50 4.5 Gold (Au) 80 2 Silver (Ag) 50 .4 Carbon 50 675 ______________________________________
It was found that the conductivity of the gold paint increases with cure temperature as supported by the specification sheet sent with the paint. However, there was concern that elevated cure temperatures might adversely alter the integrity of the coating. Therefore the lowest possible recommended cure temperature of 50.degree. C. was chosen for all the paints.
Each coupon is comprised of two wires (13, 14), a painted electrode grid (10), an the coated sample (11). One wire (14) is bonded to the metal on the backside of the coupon and acts as the working electrode. The second wire (13) is bonded to the painted grid and acts as the reference and counter electrode. This two electrode approach have thus far been tested using the AC Impedance technique with success.
The following paints have been utilized for the electrodes:
Gold Paint (FIG. 2)
The gold paint used as a painted electrode (2) was the EPO-TED H81E form Epoxy Technology, Inc. The EPO-TEK H81E is a two component, gold filled epoxy. The paint is 100% solids (solventless) and will not outgass. In addition, the gold paint has high electrical conductivity resulting in low interfacial impedance between the gold and the coating. A drawback of the gold paint is the price.
Silver Paint (FIG. 3)
The silver paint used for painted electrodes is the 102-05F electrically conductive ink form Creative Materials, Inc. The 102-05F is one part 85% silver epoxy. The epoxy has high solvent resistance and has excellent adhesion and is resistant to scratching and abrasion. The silver epoxy is a lower cost alternative to the gold epoxy.
Carbon Paint (FIG. 4)
The carbon paint used for painted electrodes is the 104-18 electrically conductive ink from Creative Materials, Inc. The 104-18 is one part 85% carbon epoxy. Even though the electrical conductivity of the carbon is less than that of the gold paint, the carbon paint still has a much greater conductivity as compared the coating to which it will be applied. Results were similar to the gold paint electrodes. Carbon conductive paint is the most cost effective alternative to the gold paint electrodes. Carbon conductive paint is the most cost effective alternative to the gold and silver paints.
Nickel Paint
Nickel paint was used a electrode material because it is inexpensive and offers high conductivity. Its corrosion resistance is greater than that of the silver paint and its abrasion resistance is greater than the carbon paint.